In-situ Monitoring the Setting Behavior of Foamed Concrete Using Ultrasonic Pulse Velocity Method

The applicability of ultrasonic pulse velocity （UPV） method to in-situ monitor setting and hardening process of foamed concrete （FC） was systematically investigated. The UPVs of various FC pastes were automatically and continuously measured by a specially designed ultrasonic monitoring apparatus （UMA）. Ultrasonic tests were performed on FC mixtures with different density （300, 500, 800 and 1 000 kg/m3）, and different fly ash contents （0%, 20%, 40% and 60%）. The influence of curing temperatures （20, 40, 60 and 80~C） was also studied. The experimental results show that three characteristic stages can be clearly identified during the setting process of an arbitrary FC paste： dormant stage, acceleration stage, and deceleration stage. Wet density, fly ash content, and curing temperature have great impact on setting behavior. A stepwise increase of the wet density results in shorter dormant stage and larger final UPV. Hydration reaction rate is obviously promoted with an increase in curing temperature. However, the addition fly ash retards the microstn,lcture formation. To aid in comparing with the ultrasonic results, the consistence spread test and Vicat needle test （VNT） were also conducted. A correlation between ultrasonic and VNT results was also established to evaluate the initial and final setting time of the FC mixtures. Finally, certain ranges of UPV with reasonable widths were suggested for the initial and final setting time, respectively.

Ceramic Setting Phosphate Binders for Bonding Refractory Concretes

The ceramic setting binders obtained by means of the raw mix mechano-chemical processing(mechanical dispersion and chemical interaction processes combination), used for bonding refractory concretes, become more available with a subsequent increased interest in their properties because they develop initial strength during a heat up of the units within 8-10 h, in contrast to 24 h after placement for high Al_2O_3 cements(HAC). This strength development enables high temperature processing units to be relined with minimum turnaround time, thus providing a favored cost/performance ratio. The present paper identifies the predominant phosphate phases responsible for developing good mechanical-strength properties of zirconia and alumina concretes bonded with mechano-chemical binders. It sets out the colloid and crystalline phases resulted from interphase interaction that provide the basis for high concentrated bonding suspensions(HCBS) technology useful to the refractory industry.

Testing the Setting and Hardening of Concrete Using the Prism Technique

In non-destructive testing of concrete, the ultrasonic pulse-echo technique has proven to be a reliable method for locating cracks and other internal defects. An electro-mechanical transducer is used to generate a short pulse of ultrasonic stress waves that propagate into the object being inspected. Reflection of the stress pulse occurs at boundaries separating materials with different densities and elastic properties. The reflected pulse travels back to the transducer that also acts as a receiver. The received signal can give many insights to the properties of materials under test. The question arises how the setting and hardening of cement paste, mortar, and concrete can be measured continuously. Continuous measurement asks for non-destructive methods. The current testing method such as the vicat needle for cement paste and penetration resistance test for concrete methods measure at intervals. These methods can be applied before the end of setting. All these methods do not allow continuous measurements and axe partly destructive In this paper a special pulse-echo technique called ＂prism technique＂ is used to evaluate the evolution of proprieties of concrete over time. For that, an automatic system has been developed with LabVIEW program in order to monitor the time of flight of the reflected p-wave. Two mixtures of mortar are made using different proportions, of water, cement, and sand. The mortar is poured into a mould that has a form of a prism. Measurements are taken every 10 minutes over a period of 24 hours. This enabled us to plot the evolution of p-wave velocities over time for the samples and compare the results.

A Success Set in Concrete

STARS seemed to have aligned perfectly for Chinese company China Wu Yi Co. Ltd. in recent years, At the firm＇s Kenya Branch office in Nairobi, an eye- catching map hanging on the wall is covered with little red dots. each one representing a project undertaken by the company in the East African country.

Calculation Model of Equivalent Strength for Induced Crack Based on Double-K Fracture Theory and Its Optimizing Setting in RCC Arch Dam

By means of fracture testing on roller-compacted concrete （RCC） three-point bending beams with two different specimen sizes, the P-CMOD complete curve for RCC was gained. Furthermore, by applying double-K fracture theory, KiniⅠC,KunⅠC, as well as the critical effective crack length and the critical crack tip opening displacement, were evaluated. Based on the double-K fracture parameters above, the calculation model of equivalent strength for induced crack was established, thus the calculation method on its initiation, stable propagation and unstable fracture was ascertained. Moreover, the finite element simulation analysis of stress field in ShaPai arch dam and the on-site observational splaying points of induced crack at different altitudes validated the reliability of the model. Finally, crack inducer′s optimal setting in RCC arch dam was studied. It improves the design level of induced crack in RCC arch dam and satisfies the necessity of engineering practice.

Effects of Wood Ash on Properties of Concrete and Flowable Fill

This research investigated the use of wood ash to partially replace cement or sand in conventional concrete, roller compacted concrete (RCC), and flowable fill. The main focus was to determine how the wood ash addition affected the main fresh and hardened properties of these materials. It was found that the wood ash could be successfully incorporated into the conventional concrete. In particular, the wood ash addition not only accelerated the setting, but also improved the early and the 28-day compressive strength of concrete that contained the blast furnace slag. It was also observed that the wood ash could be positively added into RCC to facilitate the compaction and reduce the risk of segregation. In addition, the wood ash can be beneficially introduced into the flowable fill mixtures to facilitate flow, to alleviate bleeding and subsidence, as well as to achieve controlled strength especially when combined with the class C or the class F fly ash.

Hydration Process and Crack Tendency of Concrete Based on Resistivity and Restrained Shrinkage Crack

Hydration process, crack potential and setting time of concrete grade C30, C40 and C50 were monitored by using a non-contact electrical resistivity apparatus, a novel plastic ring mould and penetration resistance methods, respectively. The results show the highest resistivity of C30 at the early stage until a point when C50 accelerated and overtook the others. It has been experimentally confirmed that the crossing point of C30 and C50 corresponds to the final setting time of C50. From resistivity derivative curve, four different stages were observed upon which the hydration process is classified; these are dissolution, induction, acceleration and deceleration periods. Consequently, restrained shrinkage crack and setting time results demonstrated that C50 set and cracked the earliest. The cracking time of all the samples occurred within a reasonable experimental period thus the novel plastic ring is a convenient method for predicting concrete＇s crack potential. The highest inflection time（t_i） obtained from resistivity curve and the final setting time（t_f） were used with crack time（t_c） in coming up with mathematical models for the prediction of concrete＇s cracking age for the range of concrete grade considered. Finally, an ANSYS numerical simulation supports the experimental findings in terms of the earliest crack age of C50 and the crack location.

Preparation for Retarding and High Early Strength Concrete

The primary objective of this research was to determine optimum dosage of mixing concrete containing plasticizers and fly ash, consistent with desirable structural grade concrete properties. Factorial tests were also conducted to investigate the four main factors： water-cementing materials ratio, water content, content of superplasticizers （SP） and fly ash content. It was found that the requirement for setting time played the dominant role in shrinkage and anti-cracking, and fly ash played a critical role in workability and reducing heat of hydration but showed insignificant effects on slump, early strength and initial setting time of concrete.

Experimental study on retarding mortar and its application in prestressed concrete

Based on a large number of orthogonal tests and theoretical analyses, the retarding mortar which meets the requirements of retard-bonded prestressed concrete was prepared. Initial setting time of the retarding mortar may vary from several hours to 15 d at 5 ℃-35 ℃ due to quantities and average curing temperature. And its 28 d compressive strength is above 35 MPa. Thus the influence of quantities on setting time and 28 d compressive strength, and the relationship between setting time and average curing temperature were investigated. The optimum quantities were obtained by studying the interaction of admixtures, and the retarding mechanism was discussed. Based on 52 retard-bonded prestressed strands by manual work from 24 retard-bonded prestressed concrete T-beams, static friction drag, change factor κ and friction factor μ were obtained from the test when retard-bonded prestressed strands were tensioned. Application of the retarding mortar will be vast in practical concrete projects.

Workability Tests on Fresh Concrete Formulated with Eco-friendly Admixture

Extra-cellular compounds, secreted by microorganisms into their surroundings, can be integrated in concrete composition as admixtures. These substances are important in biofilm formation and some of them can be used as corrosion inhibitor of concrete reinforcement. This paper deals with products made with biological surface active compounds/agents allowing the development of more eco-friendly concrete. The influence of this environmentally friendly bio admixture on setting time, workability, bending and compressive strengths of various mortar based materials made of CEM I, CEM III and CEM V was studied. Mechanical tests were carried out to highlight the influence of admixture in workability and hardening of samples containing the biological product with ratio in the range of 0-2.5%. It was demonstrated that the presence of the new bio-compound admixture in mortar decreases their compressive strength after 28 days of standard curing, in spite of remaining higher than standard minimal strength. Furthermore, Vicat needle experiments have shown a tendency of this admixture to decrease the setting time. A discussion was finally proposed in order to correlate the setting times and the decrease of the mortar compressive strength, corresponding in fact to a hardening delay. This setting time delay could be linked to a delay of the admixtured mortar to increase its resistance. The slump results highlight the action of bio-admixture as a plasticizer on mortars because it increases their workability for a same water-cement ratio. This effect seems variable according to the added amount.

Influence of Selected Curing Techniques on Compressive Strength of Concrete From Palm Kernel Shell Ash and Ordinary Portland Cement

This paper discusses the findings of an experimental study on the effect of various curing procedures on the compressive strength of concrete pro­duced by partially substituting portland cement with Palm Kernel Shell Ash(PKSA).Palm kernel shell ash was utilized in a 1:2:4 mix ratio as a partial substitute for ordinary Portland cement(OPC)at percentage levels of 0%,10%,and 15%.River sand with particles passing a 4.75 mm BS sieve was used,as well as crushed aggregate with a maximum size of 20 mm,and palm kernel shell ash with particles passing a 212μm sieve.The compressive strength of the test cubes(150 mm × 150 mm × 150 mm)was determined after 7,28,and 56 days of curing.The results demonstrated that test cubes containing Palm kernel shell ash developed strength over a longer curing period than ordinary Portland cement concrete samples and that the strength changes depending on the amount of PKSA in the cube samples.The findings showed that at 28 days,test cubes with 5%,10%,and 15%PKSA content in all curing procedures utilized obtained a greater compressive strength.Curing by immersion produced the highest compres­sive strength in all replacement level while the concrete cured by sprinkling and spraying gives a lower strength in all replacement level.

A Condition States Assessment System for Concrete Bridges Using Neural Networks

Due to continuing aging and heavy utilization of many bridges and the limited available funds, the importance of proper bridge condition state assessment has risen recently, which is the crucial point for rational decision-making on MR&R activities. This paper presents a prototype of the concrete bridge condition state assessment system (CBCSAS) with the following sub-modules: inspection, parameter recognition, structural assessment, main cause identification and priority-to-action. And multi-layer neural networks, which may combine with fuzzy set theory or not, are performed to realize the structural assessment with embedding expert knowledge into the assessment system.

水下不分散混凝土抗分散剂优化设计与凝结时间调控

水下不分散混凝土(non-dispersible underwater concrete,NDC)为涉水工程的施工提供抗冲刷能力,为实现水下不分散混凝土性能的提升,文章探讨抗分散剂的优化设计和凝结时间的调控,选取聚丙烯酰胺(polyacrylamide,PAM)和羟丙基甲基纤维素(hydroxypropyl methyl cellulose,HPMC)作为抗分散剂主剂,聚羧酸减水剂、有机硅和聚醚类消泡剂作为辅剂,从抗分散性和流动性的角度对水下不分散混凝土抗分散剂的组成进行优化设计,同时选取碳酸锂和硫酸铝作为调凝剂,对水下不分散混凝土的凝剂时间进行调控。研究结果表明:采用HPMC作为主剂、聚羧酸减水剂和聚醚类消泡剂作为辅剂制备抗分散剂,可以使得水下不分散混凝土具有较好的抗分散性和流动性;其中,含水泥质量分数0.45%的HPMC和1%聚羧酸减水剂的新拌水泥砂浆(水胶质量比为0.5,胶砂质量比为2)表现出优异的抗分散性和良好的流动性,其pH值为9,扩展度为150 mm;在该体系中,硫酸铝的调凝效果较好且经济效益更高,但使用时用量应少于2%。对抗分散剂的优化设计和凝结时间的调控为水下不分散混凝土的相关实验研究提供了重要参考。

基于沥青砂涂层设计的沥青混凝土路面抗紫外光老化性能分析

针对沥青路面光老化问题,参照含砂雾封层技术原理,基于沥青抗紫外光添加剂最优组合,制备含砂沥青涂层,开展不同级配和厚度的沥青砂涂层试验研究,通过沥青混凝土高低温性能试验和抗滑性能试验分析含砂涂层设计下沥青混凝土路面抗光老化性能,结果表明:涂层设计下沥青路面老化前后抗滑性能降低,但是相比于无涂层时老化后降低幅度变小且仍满足规范要求,在不考虑抗滑损失的情况下,可选择使用;老化后高低温路用性能试验说明增加路表面密实度可以提高抗光老化能力,但是涂层厚度不宜过厚,以基本完全填充混凝土表面空隙最好,建议理论厚度2 mm,要重点考虑细料涂层的级配设计,适当提高沥青砂中较大粒径的比例对封层设计效果会更好。

混凝土用新型缓凝剂的制备及其性能评价

为了有效延长混凝土的凝结时间,提高混凝土的长距离泵送性能,以烯醇基聚氧乙烯醚TPEG、甲基丙烯酸MAA和丙烯酸磷酸酯单体FM-1为主要原料,通过自由基聚合的方式制备出了一种适合混凝土用的新型缓凝剂GRE-1,并对其综合性能进行了评价。试验结果表明:新型缓凝剂GRE-1的掺入能够有效改善水泥浆的流动性能,延长水泥浆的凝结时间,当其质量浓度为0.5%时,可大幅提高水泥浆的流动度,并能使水泥浆的初凝时间和终凝时间延长一倍以上;GRE-1还能与水泥浆中的阳离子产生结合作用,有效降低滤液中阳离子的浓度;GRE-1的掺入能够大幅提升水泥砂浆试件的早期抗压强度和抗折强度,改善其力学性能;GRE-1在混凝土中的应用效果明显优于常用市售缓凝剂HT-1和HP-1。研究结果认为研制的新型缓凝剂GRE-1可以有效改善混凝土的工作性能,能够满足混凝土长距离泵送的使用需求。

改性高浓度硼酸溶液对蛇纹石屏蔽混凝土力学性能与中子屏蔽性能的影响

蛇纹石屏蔽混凝土是一种长期应用于高温条件下仍能保持高结晶水含量的中子屏蔽材料,但其密实度低,缺乏有效吸收热中子的硼元素。作为硼元素的来源,硼酸是生产屏蔽混凝土的有效材料,但在以水泥为主的水化体系中,硼酸的掺入会抑制水泥水化过程,导致材料强度及耐久性劣化。本研究通过改性高浓度硼酸溶液,诱导水泥浆体中游离硼酸盐生成高硼含量与结晶水含量的戈硼钙石,增加体系中硼元素含量的同时解除硼酸对水泥水化的抑制作用,研究改性高浓度硼酸溶液对蛇纹石屏蔽混凝土工作性能、力学性能、高温残余抗压强度、微观结构与屏蔽性能的影响。结果表明:掺加改性高浓度硼酸溶液对蛇纹石屏蔽混凝土的凝结时间、常温力学性能、高温残余抗压强度均无不良影响。掺加改性高浓度硼酸溶液后,蛇纹石屏蔽混凝土在400℃高温残余抗压强度提高了43.5%,常温下半值层降低了31.7%,350℃保温至恒重后半值层降低了21.4%,对中子射线的屏蔽能力增强。

基于纳米二氧化硅改性下SAP内养护混凝土的基本性能研究

研究纳米二氧化硅(NS)改性SAP内养护混凝土的物理力学性能以及抗氯离子渗透性能,提出该混凝土的配合比设计。复掺0.08%SAP和1.5%NS能提高混凝土抗折强度;复掺0.08%SAP和1.0%NS能提高其抗压强度;复掺0.24%SAP和1.5%NS氯离子抗渗透性能最佳;复掺0.24%SAP和0.5%NS能显著减小干燥收缩率。

基于AdaBoost算法的混凝土抗压强度预测

论文采集1030组混凝土抗压强度试验数据,通过训练AdaBoost算法,得到可用于预测混凝土抗压强度值的模型。结果表明:AdaBoost算法模型可以在给定输入变量的情况下准确有效地预测混凝土抗压强度;10折交叉验证决定系数R2的平均值达到0.952,平均绝对百分比误差(MAPE)达到11.39%,说明十折交叉验证具有较高准确率;AdaBoost算法与人工神经网络和支持向量机独立学习算法比较,表现出集成学习算法的优越性;讨论了AdaBoost算法模型中训练数据集数量、弱学习器类型和输入变量的数量相关因素,发现使用1030数据集的80%可以获得良好的预测结果。

可滑动钢锚箱组合索塔锚固结构力学性能试验研究

佛山富龙西江特大桥4~26号斜拉索塔上锚固采用可滑动钢锚箱组合索塔锚固结构,为掌握这种新型索塔锚固结构的力学性能,开展1∶2缩尺模型试验,对滑动副的工作性能、钢锚箱和混凝土塔柱应力状态、钢锚箱与混凝土塔柱间竖向相对滑移量、斜拉索索力传递路径及混凝土塔壁裂缝的发展规律进行研究。结果表明:镜面不锈钢与双组份石墨烯改性氟碳漆涂层组成的滑动副摩擦系数为0.055,适合作为可滑动钢锚箱的滑动副;钢锚箱侧拉板在斜拉索索力作用下处于拉、弯、剪复合受力状态,最大应力为150 MPa,应力水平较低,混凝土塔柱顺桥向最大拉应力仅为0.57 MPa,即使不设预应力也无开裂风险;钢锚箱与混凝土塔柱间的竖向相对滑移量很小,二者间连接可靠,能够很好地共同承受竖向索力;滑动端锁定前,竖向索力几乎全部由钢锚箱的固定端通过剪力连接件传递给混凝土塔柱,水平索力几乎全部由钢锚箱承担,滑动端锁定后,斜拉索索力增量由钢锚箱与混凝土塔柱共同承担;加载至2.5倍标准组合索力过程中,结构依次出现竖向裂缝和水平裂缝,所有裂缝宽度均较小,最大宽度仅0.2 mm。

Effect of soil set-up on the capacity of jacked concrete pipe piles in mixed soils

The increase in capacity of displacement piles with time after installation is typically known as soil/pile set-up. A full-scale field test is carried out to observe the set-up effect for open-ended concrete pipe piles jacked into mixed soils. Both the total capacity and the average unit shaft resistance increase approximately linearly with logarithmic time. The average increase rate for unit shaft resistance is 44% per log cycle, while the average increase for total capacity is approximately 21%. A review on case histories for long-term set-up indicates an average set-up rate of approximately 40%. Based on this, the mechanism of pile set-up is discussed in detail and a three-phase model is suggested.